that era dodge ram 50 ran one of two engines, both of which were made by mitubishi. The dodge ram 50 was an import truck despite having the dodge name. The engines offered were a 2.0 liter four cylinder and a 2.6 liter four cylinder. The 2.6 liter was a counterbalance shaft engine that is said to be very balanced to the point you can put a glass of water on the running engine and it wont sway around in the glass.
The 2.6 liter filled up most of the underhood, where the 2.0 liter left plenty of working room under the hood.
The 2.6 liter was used more than the 2.0 liter.
Hope this heads you in the right direction.
Go to an auto parts place and ask to see air filters for both engines. See which one fits and you will most likely know your engine size.